The correct option is C The assertion is true but the reason is false
Interphase was often refered to as the "resting phase" because no activity that could be seen from a light microscope occurs during the interphase. Light microscopy can only see activity when the cell is actively dividing. Hence, the "Assertion" is true.
The "Reason" is false. The interphase is the preparatory phase when cell prepares itself for the cell division. Interphase is when the cell lives its "life". The cell spends most of its life in the interphase. It obtains nutrition from the outside and metabolizes the nutrients as well.
During the interphase, the cell produces lots of proteins, it increases in size, necessary organelles organelles are duplicated and the DNA is doubled as well. All this happens as preparation for the cell division event. So, it is wrong to say that the cell is metabolically "inactive".